Process for preparing dialkylizinc compounds from alkylbromide and alkyliodide

ABSTRACT

A DIRECT PROCESS FOR PREPARING DIALKYLRINC COMPOUNDS, WHEREIN ZINC ALLOYED WITH SODIUM, POTASSIUM OR LITHIUM IS REACTED WITH A MIXTURE OF ALKYLBROMIDE AND ALKYLIODIDE IN THE ABSENCE OF MOISTURE TO PRODUCE A DIALKYLZINC COMPOUND.

United States Patent 3,641,080 PROCESS FOR PREPARING DIALKYLIZINC COM- POUNDS FROM ALKYLBROMIDE AND ALKYL- IODIDE Schrade F. Radtke, New Canaan, Conn., assignor to International Lead Zinc Research Organization Inc., New York, N .Y. N0 Drawing. Filed Jan. 21, 1969, Ser. No. 792,845 Int. Cl. C071? 3/06 US. Cl. 260-4293 13 Claims ABSTRACT OF THE DISCLOSURE A direct process for preparing dialkylzinc compounds, wherein zinc alloyed with sodium, potassium or lithium is reacted with a mixture of alkylbromide and alkyliodide in the absence of moisture to produce a dialkylzinc compound.

This invention relates to a direct process for preparing dialkylzinc compounds.

Diorganozinc compounds are generally prepared by two methods. The first method reacts a zinc halide with a reactive organomet'allic, such as lithium or magnesium, in an ether solvent. The second method involves a direct synthesis starting from metallic zinc or zinc alloys with alkyliodides. Mixtures of alkyl bromides and iodides have also previously been used in combination with zinc-copper alloy.

The first method is disadvantageous in that it requires two separate steps to be performed. In the first step the lithium or magnesium reagent must be formed, and in the second step the actual formation of the organozinc compound takes place. An additional disadvantage is the presence in the reaction of flammable solvents. The separation of the solvents from the product is not always easily achieved, especially with the utilization of lower alkyls.

The second method is more attractive in that only one step is required and the use of solvents is not necessary, although recent attempts to find improved conditions have utilized solvents with high dielectric constants, such as dimethylformamide. The use of solvents, however, makes it impossible to separate solvent from the product.

It has been suggested that the reaction of ethyliodide and the fusion product of zinc with a large proportion of sodium will result in a diethylzinc compound. The iodide starting material, however, is expensive, and the presence of free sodium in the zinc alloy is hazardous and diflicult to work with.

Since metallic zinc alone does not react with alkyliodide or bromide to produce more than minimal amounts of the dialkylzinc even under ideal conditions, attempts have been made to use a zinc-copper alloy or fusion product. Unless extreme precautions are taken, however, the reaction does not start for several hours. If, after the start of the reaction, the reaction mixture is cooled too low, the reaction may stop entirely and is difficult to begin again. More importantly it is recognized that the yields are then much lower. An additional disadvantage is the necessity of using expensive copper and alkyliodides as starting materials.

It is therefore an object of this invention to provide a method for preparing dialkylzinc compounds by direct synthesis, and more particularly to provide such a process utilizing less expensive starting materials.

According to the invention it has been found that zinc alloyed with sodium, potassium, or lithium will react with a mixture of alkylbromide and alkyliodide to synthesize dialkylzinc compounds. The reaction is believed to follow the general formula:

3,641,080 Patented Feb. 8, 1972 The foregoing illustrates the reaction when the ratio of bromideziodide is 1:1. With higher ratios, 21 similar reaction would take place.

The reactions are performed by refluxing finely divided zinc alloy with alkylbromide and alkyliodide. The reaction should be carried out in the absence of moisture, and therefore an inert atmosphere is preferred, such as nitrogen, argon, or carbon dioxide. The reactions may be carried out at atmospheric pressure, unless the starting materials are too volatile, in which case pressure should be applied. This may be necessary with mixtures of MeBr and MeI.

It is preferred to use an excess of the zinc alloy since this increases the speed and the yield of the reaction. A molar ratio of zinc to the mixture of alkylbromide and alkyliodide of between 2:1 and 1:1 has been found satisfactory. It is unnecessary to perform the reaction in the presence of a solvent, since it is usually very diflicult, if not impossible, to separate the product from the solvent.

The molar ratios of alkylbromide to alkyliodide may range from 1:1 to 20: 1. Good results were obtained using ratios of 1:1 to 5:1 alkylbromide to alkyliodide. Due to the lower cost of the alkylbromide, however, ratios of 3 :1 and above are most preferred.

The reaction usually begins a few minutes after the starting materials have made contact with each other, especially if substantially all traces of moisture have been removed from the atmosphere and the apparatus. Heating of the reaction mixture may be necessary to begin the reaction, but if the reaction is exothermal refluxing temperature is maintained without the application of external heat. In the event that the reaction is not exotherrnal, external heating may be applied to maintain the reflux temperature. It has been found that a temperature in the range of 40 -1 C. is satisfactory. A temperature range of 140 C. is preferred.

The reaction is considered complete when refluxing stops. The flask is connected with a distilling head and the contents of the flask are distilled under reduced pressure to recover the dialkylzinc compound.

The zinc alloys which have proved themselves reliable for purposes of the invention are the lithium, potassium and sodium alloys. The alloys may be formed by fusing the metals together in a steel crucible under an inert atmosphere, for example, argon. The cooled melt is machined to fine particle size. Usually turnings may be used, but if the alloy is very brittle a sandlike material is obtained upon machining. The shot method may also be utilized to prepare the alloys on an industrial scale.

The maximum amount of sodium which can be alloyed with zinc is one atom of. sodium per twelve atoms of zinc. This corresponds with an alloy containing three wt. percent sodium, the balance being zinc.

Table I contains data showing the yield of dialkylzinc in relation to the percentage of sodium in the alloy.

It is apparent that the yield of dialkylzinc decreases as the sodium content of the alloy decreases. The 2% sodium/zinc alloy gives, however, no significantly lower yields than the 3% sodium/zinc alloy. The zinc alloy containing as little as 1% sodium also gives good results with the alkylbromide. Upon further lowering of the sodium content in the zinc alloy, the yields of dialkylzinc gradually decreases to those obtained using pure zinc. In view of the higher yields obtainable with the 2% and 3% sodium/zinc alloy, it is preferred that the percentage of sodium lie between 2 and 3 wt. percent.

Zinc alloyed with potassium is obtained by fusing zinc and potassium under an inert atmosphere, such as nitrogen, and machining in the usual way. The alloy is very reactive and should be kept in an inert atmosphere.

The maximum amount of. potassium in the zinc/potassium alloy is approximately 5 wt. percent, which corresponds to a molar ratio of 12 moles zinc to 1 mole potassium. The reaction with the alkylbromide and alkyliodide mixture is carried out in the same manner as with the zinc/sodium alloy. Once begun, the reaction continues spontaneously upon addition of the bromide and iodide.

A comparison of the yield in dialkylzinc as a function of the percent of potassium in the zinc alloy is shown in Table II.

It is evident that the potassium content of the alloy may be reduced without loss of activity. The zinc/potassium alloys are easily handled and machined to small particle size. As appears from the results in Table II, the 2% potassium/zinc alloy is well suited for the direct synthesis of zinc dialkyls, although the yields in general are somewhat lower than for the zinc/sodium alloys. The recommended range would be 0.5-5.0 wt. percent potassium with 1.5 to 2.0 wt. percent potassium preferred.

Reactions with the zinc/potassium alloys started without any appreciable induction period and were easily controlled.

In formulating the zinc/lithium alloys it was found important to control closely the homogeneity of the alloy. Table III illustrates reactions with zinc/lithium alloys.

It was found that a useful lithium range is -1 to wt. percent lithium in the zinc alloy, with a preferred content of approximately 2. wt. percent lithium.

Ternary alloys may be used to provide good yields of dialkylzinc compounds. The amalgamation of the zinc/ sodium alloy by treating the 3% sodium/zinc alloy with HgCl in tetrahydrofuran provides approximately the same yield as using the zinc/sodium alloy without the HgCl An alloy composed of 2.6% sodium, 1.1% mercury and the rest zinc was reacted with a 1:1 ratio of EthBr/ EthI and yielded 71% diethylzinc.

The synthesis according to the invention may be carried outwith both normal and branched chain alkylbromides and iodides. Since longer chain dialkylzincs, for example, where R is greater than C have limited thermal stability, a direct synthesis is not practical. At the other end of the scale, the boiling points of methylbromide and methyliodide are low and require reaction under higher pressure conditions than a mo pheri p re.

The alkyl group may be unsaturated, but the double bond should be more than two carbon atoms removed from the zinc atom. Both Zn(CH CH=CH- and Zn(CH CH=CHCH are thermally unstable. Compounds With a CEC group "can be prepared if the CEC group is internal. Compounds with a terminal CECH group decompose due to the acidity of the terminal hyd-rogen.

The following examples illustrate the process of the in vention and are not intended to limit in any way the scope of the invention.

EXAMPLE 1 Et Zn from Zn/Li alloy (2% Li) and EtBr/EtI 1:1

To 63 g. of Zn-2.0 Li (0.96 g. atom Zn) a few ml. of a mixture of 18 ml. of EtBr (0.24 mole) and 10 ml. of EtI (0.24 mole) was added. The mixture was gradually heated (to oil bath temp.-) and an exothermal reaction started. The remainder of the halide mixture was added in 35 min. After a further 20 min. heating period (oil bath-) the diethylzinc was isolated by distillation. Yield: 22.9 g. (77% of theory).

EXAMPLE 2 Et Zn from Zn/Na/Hg alloy and EtBr/EtI 1:1

The reaction of 58.3 g. Zn-2.6 Na-1.1 Hg (0.84 g. at Zn) and a mixture of 16 ml. EtBr (0.21 mole) and 17 ml. of EtI (0.21 mole) carried out as in Example 1 afforded 18.5 g. (71% of theory) of Et Zn.

EXAMPLE 3 n-Bu Zn from Zn/ K alloy (2% K) and BuBr/BuI 1:1

Upon addition of a few ml. of a mixture of 10.5 ml. BuBr (0.10 mole) and 11 ml. of BuI (0.10 mole) to 29.3 g. of Zn-2.0 K (0.40 g. at. Zn) an exothermal reaction started. The remainder of the halide mixture was added dropwise. Finally the reaction mixture was heated at 130 for 40 min. 11.2 g. of n-Bu Zn (63% of theory) were isolated by fractional distillation.

EXAMPLE 4 Et Zn from Zn/K alloy (2% K) and EtBr/EtI 18:1

A mixture of 27 ml. of EtBr (0.35 mole), 1.0 ml. of EtI (0.10 mole) and 48 g. of Zn-2.0 K (0.70 g. at. Zn) was heated at reflux temperature with vigorous stirring (oil bath-85). After an induction period of several hours the reaction proceeded normally. After fractional distillation 11.5 g. (53% of theory) of Et Zn were isolated.

EXAMPLE 5 Et Zn from Zn/Na alloy (3% Na) and EtBr/EtI 5:1

-In a 500 ml. three-necked flask (condenser, stirrer, dropping funnel, N atmosphere) is placed 14.7 g. finely divided Zn-3.0 Na (220 mmole) alloy and 11.8 g. (107 mmole) EtBr. The mixture is heated under reflux (oil bath 120) and 3.1 g. EtI (20 mmole) is added to the refluxing mixture. An exothermal reaction occurred (with some foaming) which was completed after 20 min. The mixture completely solidified on cooling. The flask was connected with a distilling head, cooler and receiver. Et Zn was distilled under reduced pressure (circa 2 mm. Hg; temp. oil bath circa into the distilling flask which was kept in acetone Dry Ice. The yield of Et Zn was 5.8 g. (85% of theory).

EXAMPLE 6 Et Zn from Zn/Na alloy (1% Na) and EtBr/EtI 3:1

In a 500 ml. three-necked flask were placed 58 g. finely divided Zn-1.0 Na (0.45 mole) and 8 ml. of a mixture consisting of 25 ml. EtBr (0.33 mole) and 9 ml.

of EtI (0.11 mole). The contents of the flask were heated (oil bath at 80100) and an exothermal reaction started. The remainder of the halide mixture was added dropwise. After a 90 minute reflux period Et Zn was isolated by distillation as described in Example 5. The yield of Et Zn was 19.8 g. (70% of theory).

EXAMPLE 7 Et Zn from Zn/ K alloy K) and EtBr/EtI 3 :1

To 55.0 g. finely divided Zn-6.6 Li alloy (0.78 g. at. Zn) kept in a 500 ml. flask a few ml. of a mixture of ml. EtBr (0.195 mole) and 5 ml. EtI (0.065 mole) were added. When the mixture was heated to reflux temperature an exothermal reaction started immediately and the remainder of the halide mixture was added dropwise. After keeping the mixture at 100 for one hour volatile products were removed under reduced pressure. Refractionation of the volatile fraction aflorded 9.8 g. of E gZl'l (62%) of theory.

EXAMPLE 8 Et Zn from Zn/ Li alloy (6.6% Li) and EtBr/EtI 5:4

To 55.0 g. finely divided Zn-6.6 Li alloy (0.78 g. at. Zn) kept in a 500 ml. flask a few ml. of a mixture of 19 ml. EtBr (0.25 mole) and 17 ml. EtI (0.20 mole) were added. When the mixture was heated at reflux temperature an exothermal reaction started. The remainder of the halide mixture was added dropwise. The reaction mixture was heated until refluxing stopped (about 20 minutes). Et Zn was isolated in the usual way. The yield after refractionation was 22.6 g. (82% of theory).

I claim:

1. A process for preparing dialkylzinc compounds comprising:

(a) reacting in a moisture-free atmosphere:

(i) an alloy of zinc having at least one metal selected from the group consisting of sodium, potassium and lithium, the amount of metal in the alloy comprising about 1 to about 3 weight percent sodium, about 0.5 to about 5.0 weight percent potassium and about 1 to about 10 weight percent lithium, with the remaining percentage being zinc, with (ii) a mixture of alkylbromide and -alkyliodide in a molar ratio of at least 1:1 zinc to the alkylbromide and alkyliodide mixture, the molar ratio of the bromide to iodide lying between about 5:1 and about 20:1, the alkyl radical being selected from the group consisting of saturated and unsaturated alkyl radicals containing from 1 to 8 carbon atoms; and

(b) refluxing the mixture of alloy and alkylbromide at a temperature of between 40 C. and about 180 C. until the refluxing ceases.

2. A process as described in claim 1, wherein the refluxed mixture is distilled under reduced pressure in the absence of moisture to separate the volatile dialkylzinc compound from the involatile residue.

3. A process as described in claim 1, wherein the zinc alloy is in particulate form.

4. A process as described in claim 1, wherein the zinc alloy contains approximately 2-3 wt. percent sodium.

5. A process as described in claim 1, wherein the zinc alloy contains approximately 1-2 wt. percent potassium.

6. A process as described in claim 1, wherein the zinc alloy contains approximately 2 wt. percent lithium.

7. A process as described in claim 1, wherein the zinc alloy contains both sodium and lithium in addition to ZlIlC.

8. A process as described in claim 1, wherein the alkyl radical is methyl and the alkylbromide, alkyliodide and zinc alloy mixture is refluxed under a pressure above that of atmospheric pressure.

9. A process as described in claim 1, wherein the refluxing temperature is approximately C. to C.

10. A process as described in claim 1, wherein the molar ratio of the zinc alloy to the alkylbromide and alkyliodide mixture in the mixture is approximately 2:1.

11. A process as described in claim 1, wherein dialkylzinc is added to the reaction mixture to remove traces of moisture.

12. A process as described in claim 1, wherein the zinc alloy contains approximately 1.1 wt. percent mercury.

13. A process as described in claim 1, wherein the ratio of bromide to iodide is at least 3:1.

References Cited Rieth et al., Annalen, vol. 123, pp. 245-8 (1862).

Alexeyeff et al., Compt. rendu, vol. 58, pp. 171-73 (1864).

Nesmeyanov et al., Methods of Elementoorganic Chemistry, North-Holland Publ. Co., Amsterdam, vol. 3, pp. 8 to 14, 24 and 25 (1967).

TOBIAS E. LEVOW, Primary Examiner H. M. S. SNEED, Assistant Examiner 323 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 3 080 Dated April 25 1972 Inven tor(s) Chrade F. Radtke It iscertified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 3 line '52, change "ErBr/EtI 1:1" to Y EtBr/EtI 1:1 Col. 4, line 16, change "(0.10 mole)" to (0.01 mole) 001. 5, lines 10-1-11, change "55.0 g. finely divided Zn-6.6 Li alloy (0.78 g. at. Zn)" to 36.5 g finely divided Zn-5.0 K alloy (0.26 g at. Zn)

Signed and sealed this 1st day of August 1972.

(SEAL) A'btest:

ROBERT GOTISCHALK Commissioner of Patents EDWARD M.FL11;TCHER,JR. Attesting Officer 

